Earth-Science Reviews 99 (2010) 99–124

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Earth-Science Reviews

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The Late Miocene paleogeography of the Amazon Basin and the evolution of the Amazon River system

Edgardo M. Latrubesse a,⁎, Mario Cozzuol b, Silane A.F. da Silva-Caminha c, Catherine A. Rigsby d, Maria Lucia Absy c, Carlos Jaramillo e a University of Texas at Austin-Department of Geography and the Environment, A3100, Austin TX, 78712, USA b Universidade Federal de Minas Gerais, ICB, Departamento de Zoologia, Belo Horizonte, Brazil c Universidade Federal de Mato Grosso, Departamento de Geologia, Cuiabá, MT- 78060-900, Brazil d East Carolina University, Department of Geological Sciences, Greenville, NC, USA e Smithsonian Tropical Research Institute, Balboa, Panama article info abstract

Article history: On the basis of paleontological content (vertebrates and palynology) and facies analysis from river banks, road Received 26 February 2009 cuts, and three wells, we have assigned the uppermost levels of the Solimões Formation in western Amazonia, Accepted 8 February 2010 Brazil, to the Late Miocene. The vertebrate fossil record from outcropping sediments is assigned to the Available online 16 February 2010 Huayquerian–Mesopotamian mammalian biozones, spanning 9–6.5 Ma. Additionally, we present results that demonstrate that deposits in Peruvian Amazonia attributed to Miocene tidal environments are actually fluvial Keywords: sediments that have been misinterpreted (both environmentally and chronologically) by several authors. Amazon basin Amazon River The entire Late Miocene sequence was deposited in a continental environment within a subsiding basin. The Late Miocene facies analysis, fossil fauna content, and palynological record indicate that the environment of deposition was Paleogeography dominated by avulsive rivers associated with megafan systems, and avulsive rivers in flood basins (swamps, Paleoecology lakes, internal deltas, and splays). Soils developed on the flatter, drier areas, which were dominated by fossil vertebrates grasslands and gallery forest in a tropical to subtropical climate. palinology These Late Miocene sediments were deposited from westward of the Purus arch up to the border of Brazil with Solimoes Formation Peru (Divisor Ranges) and Bolivia (Pando block). Eastward of the Iquitos structural high, however, more detailed studies, including vertebrate paleontology, need to be performed to calibrate with more precision the ages of the uppermost levels of the Solimões Formation. The evolution of the basin during the late Miocene is mainly related to the tectonic behavior of the Central Andes (∼3°–15°S). At approximately 5 Ma, a segment of low angle of subduction was well developed in the Nazca Plate, and the deformation in the Subandean foreland produced the inland reactivation of the Divisor/ Contamana Ranges and tectonic arrangements in the Eastern Andes. During the Pliocene southwestern Brazilian Amazonia ceased to be an effective sedimentary basin, and became instead an erosional area that contributed sediments to the Amazon fluvial system. At that time, the lowland fluvial systems of southwestern Amazonia (the Purus, Jurua and Javarí basins) become isolated from the Andes by the newly formed north- flowing Ucayali system and south-east flowing Madre de Dios System. It was during the early Pliocene that the Amazon fluvial system integrated regionally and acquired its present appearance, and also when it started to drain water and sediments on a large scale to the Atlantic Ocean. © 2010 Elsevier B.V. All rights reserved.

Contents

1. Introduction ...... 100 2. The Solimões Formation ...... 101 2.1. Sedimentology revisited ...... 105 2.2. Fossil fauna ...... 106 2.2.1. Chronology ...... 106 2.2.2. The vertebrate diversity ...... 106

⁎ Corresponding author. Tel./fax: +1 512 471 5116. E-mail address: [email protected] (E.M. Latrubesse).

0012-8252/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2010.02.005 100 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

2.2.3. Summary of diet habits of mammals as environmental indication ...... 110 2.2.4. Freshwater mollusks and ostracods ...... 111 2.3. Palynology ...... 112 2.3.1. Biostratigraphic data ...... 112 2.3.2. Paleoenvironmental considerations ...... 114 3. Amazonia during the Late Miocene ...... 115 3.1. Correlation of the Solimões Formation with others lithostratigraphic units of South America ...... 115 3.2. The Amazon basin: evolutionary approach ...... 116 4. Conclusions ...... 121 Acknowledgments ...... 122 References ...... 122

1. Introduction Campbell et al., 2006. Frailey et al., 1988) with large discrepancies. The only late Tertiary lithostratigraphic unit defined and outcropping in Neogene and Quaternary paleogeographic reconstruction of Ama- western Brazilian Amazonia is the Solimões Formation, which extends zonia has produced many hypotheses and models (eg. Hoorn 1994b; for thousands of square kilometers to the west of Manaus (Fig. 1).

Fig. 1. Isopach map of the Solimões Formation (from Maia et al., 1977) showing the main depocenters and structural highs within the basin. Numbered circular areas indicate main areas with fossiliferous, palynological and /or geological data. 1 = Upper Acre river; 2 = Acre river upstream of Rio Branco; 3 = BR 364 from Rio Branco to Sena Madureira; 4 = BR 364 from Sena Madureira to Manuel Urbano and outcrops along the Iaco and Purus rivers; 5 = Upper Jurua River; 6 = Feijó-Tarauacá area; 7 = Madre de Dios basin. See Table 1 for details. Wells are indicated as 1AS-32-AM, 1AS-27-AM 1AS-19-AM and IAS-4a-AM. E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 101

Several names have been assigned to this unit in western Brazilian and Table 1 Peruvian Amazonia through time. Caputo et al. (1971) combined the Main studied areas and localities in south-western Brazilian Amazonia. previously defined Puca, (Steinmann 1929), Solimoes (Rego, 1930) Area Main Geology Fossil Palynoloy Location and Cruzeiro (Oppenheim, 1937) into a single unit defined as the localities fauna Solimões Formation. Recently, the old name Pebas (Gabb, 1869) Upper Acre River Patos X X X 10°55′00″S and applied to tertiary sediments of Peru had been retaken by several 69°55′00″W authors (Hoorn, 2006a, Räsänen et al., 1998, Wesselingh et al., 2002 Barranco da XXX10˚56′25″S and ˚ ′ ″ among others) that extended this nomenclature to sediments of the Elizete 69 45 44 W Calvancante X X – 10055′42 S and Solimões Formation in Brazil. 690 49′53 S Hoorn (1993) studying a well and outcrops from the Solimões and Cachoeira XX– 10°56′22″S and Japurá Rivers in the area close to the border of Brazil with Peru and Bandeiras 69°20′37″W) Colombia (Fig. 1) postulated that marine incursions could have Acre River Preventorio X X – Rio Branco Urban (upstream of Area-left bank reached this part of the Amazon during the middle Miocene. Räsänen Rio Branco city) et al. (1995) proposed that the upper Solimões Formation outcrop- Amapa X –– 9037′17.25 S and ping in Acre indicated the existence of a late Miocene intracontinental 68014′12 W seaway through western Amazonia and suggested that this seaway Murici X – X57′24″S and ′ ″ could have connected the Caribbean Sea with the South Atlantic, 69°44 44 W Niteroi X X X 10°08′14″S/ crossing the Venezuelan/Colombian Llanos to the Paraná basin in 67°48′46″W Argentina and passing along western Amazonia and the Beni–Chaco Purus River Talismã X X – 080 10′ 32.1 S and plain. The seaway proposal was well received by a broad community 700 29′4.1 W of researchers, mainly biologists and paleontologists, as a paleogeo- BR 364 road cuts Several XXIn See Fig. 1 (areas graphic alternative for understanding biogeographic patterns in South from Rio Branco along the analysis 3,4 and 6) to Tarauacá road America (Webb, 1996). The seaway model received some criticism (Hoorn, 1996; Marshall and Lundberg, 1996; Praxton et al., 1996), but no concrete evidence invalidating the model has been presented until uppermost portions of this formation (Santos and Silva, 1976). On the now. Recent papers by Hovikoski et al. (2007a,b) also argue for the basis of such maps, Frailey et al. (1988) postulated the existence of a existence of marine influence in southwestern Amazonia (Peruvian late Pleistocene Amazonas Lake, which they suggested, could have and Brazilian Amazon) and for the existence of an epicontinetal covered all of the Amazon Basin and part of the Orinoco Basin embayment of continental dimensions. (forming a lake larger than the Mediterranean Sea). They also Recently, detailed studies have permitted us to better estimate the hypothesized that the sediments of the Solimões Formation could age of the uppermost levels of the Solimões Formation and to be Pleistocene in age. Kronberg et al. (1991) reinforced the interpret the paleogeographic and paleoecological conditions in Pleistocene idea when they published the results of radiocarbon southwestern Amazonia. Latrubesse et al. (2007) demonstrated the dating on carbonates from the Solimões Formation at the Niteroi non existence of an intracontinental seaway during the late Miocene. fossiliferous site on the Acre River. Their dates indicated a late Recent palynological data (Silva, 2004 and this paper) provide Pleistocene age (∼40,000 to 50,000 yr. BP) and they postulated a dry additional evidence about the paleoecological conditions in the area. period during which lakes dried and vertebrates (mammals and In this paper, we propose an alternative model for the paleohy- reptiles) died in waterholes and swamps as a consequence of climatic drologic history of the Amazon basin (which reevaluates and changes during the Last Glacial Maximum. However, it is now improves the original model of Latrubesse, 1992; Latrubesse et al., understood that the vertebrate fossils from the Niteroi site are late 1997) and present a hypothesis to explain the paleogeography of the Miocene mammals of the Huayquerian South American land mammal Amazon fluvial basin during the Late Tertiary. This hypothesis is the fauna (Latrubesse et al., 1997; Cozzuol, 2006), not Pleistocene result of 18 years (1990–2008) of field work in western Brazilian mammals as per the interpretation by Kronberg et al (1991). Today, Amazonia as well as observations from the Bolivian territory of Pando Niteroi is considered one of the more important late Miocene in the border between Peru and Brazil and along the Madre de Dios fossiliferous localities of the southwestern Brazilian Amazonia. basin in Peru (Fig. 1). Many of the geological and paleontological After several fieldwork seasons in collaboration with the paleon- expeditions (by boat along the Jurua, Moa, Iaco, Purus, Acre, and tologists A. Ranzy, J.P. Souza Filho and J. Bocquentin, of the Federal Madre de Dios rivers and by car along the scarce roads of the region) University of Acre, Latrubesse (1992) postulated that the Solimões that resulted in the data described here were done in collaboration Formation strata outcropping in southwestern Brazilian Amazonia are with the paleontologists of the Federal University of Acre. For a single unit, Neogene in age, that was deposited during the late palynological analysis, samples were taken from outcrops on the Miocene and/or early Pliocene. Purus and Acre rivers and from wells IAS-32-AM, IAS-19-AM and IAS- Definitive evidence for this interpretation comes from the 27-AM (Fig. 1), drilled by the Brazilian Carbon Project in the 1970s discovery of Late Cenozoic fossil mammals, both in the lowermost (and stored at the National Department of Mineral Production, DNPM, levels of the Solimões Formation, outcropping along river banks in the Manaus, Brazil). Table 1 shows some of the main Miocene localities in region, and in its uppermost levels (which cap the hilly landscape of southwestern Amazonia. southwestern Brazilian Amazonia). The total thickness of the sedi- ments seen in outcrop ranges from ∼40 to 70 m (Latrubesse, 1992; 2. The Solimões Formation Latrubesse et al, 1997; Souza Filho and Latrubesse, 1992)(Fig. 2). Neogene strata, principally sandstones, siltstones, and claystones The Solimões Formation, defined originally by Rego (1930), of continental origin, cover almost all of western Amazonia. A detailed comprises a single lithostratigraphic unit in Brazil, outcropping in review of the Cenozoic sedimentary units defined in Brazilian the upper Solimões area; the reference section was a 570-m-deep Amazonia can be found in the publications of the Radambrasil Project sequence from well 2-RCST-1-AM at Rio Curuça (Caputo et al., 1971). (Radambrasil, 1976, 1978). The sediments of the Solimões Formation lie in horizontal to sub- The Solimões Formation overlies the Ramón Formation (lower horizontal beds and can reach thickness of over 1000 m along the Tertiary?), the oldest Tertiary unit recognized in wells near the border border between Acre and Amazonas states (Fig. 1). For many years, of Brazil and Peru. The Ramón Formation overlies the Brazilian the Brazilian geologic maps indicated a Pleistocene age for the Precambrian shield in the peripheral areas. It could cover the 102 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Fig. 2. Schematic map (A) and section (B) showing the characteristic main elements of the landscape in southwestern Brazilian Amazonia (lowlands). The grey shady area of A–B cross section in figure A shows schematically the old and lower terraces in figure B. Late Miocene sediments of the Solimões Formation outcrop along the banks of main rivers and along the watershed areas, mainly being exposed on road cuts. The Quaternary alluvial belts are composed of very fragmented older terraces (upper and middle terraces) and a well- developed lower complex terrace formed by upper Pleistocene (middle Pleniglacial/ lower upper Pleniglacial), Lateglacial, and Holocene deposits (Latrubesse, 2003, Latrubesse and Ranzy 1998, Latrubesse and Kalicki 2002 among others). Panel B is a schematic of section A–B in panel A.

Cretaceous Alter do Chão Formation to the east and the Mesozoic backwater pounding. These kinds of fine-grained deposits indicate a rocks of the Divisor Range to the west. relatively flashy behavior, with peaks of high suspended sediment The sediments analyzed here come from outcrops along the banks transport and rapid drops of water stage or slack-water effects, and of the Acre, Iaco, Purus, Jurua and Moa rivers and from outcrops along are common throughout the Solimões Formation. the roads of the region (Table 1). Our discussion is focused on the The channel assemblage can be interpreted as indicative of active uppermost ∼80–100 m of the Solimões Formation. and unstable channels with typical sequences of fining-upward In general terms, these sediments can be categorized into two sediments and cut-and-fill (Fig. 4 and 5). The presence of non- main facies assemblages: a channel-dominated assemblage and a repetitive scroll structures (epsilon cross-bedding) and abandoned floodplain/low-energy assemblage. The channel assemblage is com- channels could indicate that avulsion was an important mechanism of posed of red-brown to brown silty to clayey sands and fine-grained, adjustment of the channels. These sedimentary environments of the mainly intraformational, gravel formed by mud-balls (Figs. 3–5). Solimões Formation resemble the facies architecture of the continen- Lateral accretion structures, abandoned channels, and characteristic tal Cumberland marshes as described by Farrell (2001). trough cross-bedding dominate this facies assemblage. The floodplain/low energy assemblage is composed mainly of Fine sediment-dominated point bars are represented by large green to gray-green and blue-green clays and silty clays and is inclined sets (some decimeters thick), of fine-grained sand, with interpreted as deposited in a floodplain–lacustrine–swampy envi- predominantly type B ripple drift laminations, in which foresets are ronment. The sediments are characterized by laminations and commonly preserved. The preserved sets (typically ∼5 cm thick) convolute beds. Massive mottled beds, with bioturbation, are also indicate high contemporaneous sedimentation rates. Interbedded, characteristics. Fine laminated to massive clay plugs can be found thick, massive, muddy beds averaging 10 to 20 cm thick suggest overlying paleochannel features and paleosols; rizo-concretion and E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 103

Fig. 3. Patos locality: (10°55′00″S/69°55′00″W area 1 of Fig. 1, located at 10°55′55 S and 69°5′20 W) is a riverbank outcrop along the Acre River approximately 38 km upriver of the town of Assis Brasil. The complete succession comprises approximately 30 m. However, only the lower 6–8 m are visible and just the lower levels bear fossils. The base of the section is formed by a massive clay and clay/silt unit, red to green, visible only during low-water river stages of the dry season. A few well preserved specimens were found there. The clay/ silt beds are incised by a fluvial channel sequence with a conglomerate, less than a meter thickness of intraformational clay-ball pebbles, followed by cross-bedded sand with medium to coarse grain size. The whole channel unit is a fining-upward succession with the conglomerate and cross-bedded sands, approximately 3 m thick (1), overtopped by a clay and silt, mostly massive unit with occasional sand or conglomerates lenses (2). The conglomerates are the source of almost all the fossils (asterisk represents the position of the fossil-bearing levels) of this locality.

Fig. 4. KM 29; Road outcrop between Feijo and Tarauaca, in Acre State (area 6 of Fig. 1) (coordinates 08°10′32.1S, 70°29′4.1W). The section, 14 m thick, is characterized by a stacked sequence of channel (2 and 4) and floodplain deposits (1 and 3) containing floodplain fine sediments, mostly clays and silts with some beds of intraformational clay pebbles. Channel sediments are dominantly sand/clay with an intraformational conglomeratic bed of clay-ball pebbles at the base of unit 2. Two levels with “in situ” fossil vertebrates are identified (asterisks in the stratigraphic column) at 1.85 and 8 m from the base of the section (see Table 3). 104 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Fig. 5. Outcrop along the BR 364 road from Rio Branco to Sena Madureira (Area 3 of Fig. 1), showing channel macroform (CHD) with crevasse-splay or small delta deposits in a water- saturated environment. A wet crevasse or small lacustrine delta channel deposit (C/DD) is cutting and entering lacustrine/marshy deposits (LD). (A) Fine laminated lacustrine deposits with the presence of muddy ripples (LD); (B) Intraformational conglomerate indicating flash entrance and/or reactivation of the splay/delta; (C) Sandy sets of the channel environment with ripples (CHD).

mudcracks are common through the assemblage (Figs. 3–7). There that acted as a pounded lake. Although it seems possible that one are minor differences in color, from greenish gray to green to light might also find major deltas that enter large lakes in this environment red. With the exception of a few localities, we do not find high we did not find facies with widespread lateral continuity. In these carbon-lignite levels in these outcrops, which suggest that produc- outcrops and wells we found relatively small deltaic deposits that tion of organic matter in the swampy–lacustrine environments was entered saturated backswamp-lake environments and channel low, or that the decay of the organic matter was rapid. However, deposits. Also we found no evidence of a large single and lake- lignite horizons are frequently found in the well records of the depth basin. Flood-basin deltas are common today in modern flood Carbon Project (Maia et al., 1977) in some parts of the basin, basins (Farrell, 2001) and in the saturated floodplains of large tropical particularly in lower levels of the Solimões Formation. Paleosols and rivers, e.g., the middle Paraná River, the Pantanal, the Magdalena pedogenic features are frequent in floodplain deposits of the River, and the Amazon River, extending along many kilometers Solimões Formation (Latrubesse et al., 2007)(Fig. 5), their occur- (Smith, 1986; Latrubesse and Franzinelli, 2002; Assine and Soares, rence conforming with the existence of broad floodplains. Paleosols 2004; Assine, 2005)(Fig. 8). are often found in channel ribbon sequences in subsiding foreland X-ray diffraction analysis of the total sedimentary fraction from basins dominated by fluvial systems. This kind of sedimentary samples from the Iaco River, the Acre River (upstream of Sena sequences, also rich in fossil vertebrates have been deposited less Madureira and Preventorio locality in Rio Branco), the Purus River extensively and in drier environments than the Amazon in the Asian (close to Sena Madureira), and road cuts between Sena Madureira and Siwalik sequences (e.g. Beherensmeyer and Tauxe, 1982; Beherens- Rio Branco indicate that, in general, quartz is dominant and the meyer, 1987; Willis and Behrensmeyer, 1994; Bhatia 2003; Jain and quartz:feldspar ratio oscillates between 16:1 and 6:1. The clay Sinha, 2003; Kumar et al., 2003). minerals are similar to those recorded today in modern sediments Fossil bones (assemblages described in detail in section 3), gypsum of the Solimões-Amazon River floodplain (Johnson and Meade (1990) veins, and calcareous concretions are common in both channel and composed of a variable proportion of smectite, kaolinite, and illite. floodplain/low-energy assemblages (Figs. 3, 4 and 7). Sediments Smectite is the dominant clay mineral in the original deposits. It can deposited in the floodplain–lacustrine–swampy environments dom- be allochtonous or produced by weathering of volcanic ash in the inate the outcrops of the Solimões. lithic fragments, which can undergo marked reduction when stored Transitional facies assemblages are also present in the Solimões on the floodplain, as shown by Johnson and Meade (1990) in recent outcrops. These are represented mainly by two groups: classical floodplain sediments of the Solimões-Amazon River coming from the crevasse-splay deposits and flood-basin delta deposits (Fig. 5). Andes. Kaolinite is abundant in some samples and generally, which Crevasse-splays, formed mainly by rapid deposition of sandy suggests a secondary origin, perhaps explaining the high quartz: sediment on relatively dry floodplain surfaces, are characterized by feldspar ratio. Muscovite is present and illite is very rare. Gypsum and the interfingering of cross-bedded silt and sand with floodplain calcite, when present, are abundant, although they can both be sediments that have undergone partial or incipient pedogenesis, as diagenetic minerals or produced by pyrite oxidation. indicated by rhizoconcretions, high bioturbation, and/or mottled Similar fine sediments related to lacustrine and floodplain texture. deposits, channel deposits of sandy sediments and intercalations of The flood basin deltaic deposits are the result of the deposition of some lignite levels have been recorded as well in IAS-32-AM, IAS-19- crevasse-splay sediments in a saturated backswamp environment AM, and IAS 27-AM (Fig. 9). E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 105

The tidal environment interpretations are based mainly on the existence of flaser bedding, climbing ripples, rhythmic beds of fine/ sandy deposits, mud balls, and convoluted bedding. However, these kinds of sedimentary structures and sequences are clearly identifiable today in many modern rivers (without tidal influence) (Ray, 1976; Farrell, 1987) and continental flood basins associated with fluvial systems (Farrell, 2001), including the modern rivers in the study area. These structures are not characteristic of many large estuary systems, such as the Rio de La Plata and the modern Amazon estuary (Vital and Stattegger, 2000; Cavallotto et al., 2004). Antoine et al. (2003), Hovikoski et al. (2005), and Roddaz et al. (2006) described sedimentary sequences in the Madre de Dios River, Peru, and in the Beni River, Bolivia (Fig. 1) that they interpreted as late Miocene tidal deposits. These studies, however, had no biostrati- graphic support. In the Madre de Dios basin, the sections at Cerro Colorado (12°34′00.45811″S; 70°06′18.58780″W) (Fig. 10) and Los Amigos (12°33′26.29793″S; 70°05′29.45067″W) (Fig. 11) are de- scribed by Antoine et al. (2003) and Hovikoski et al. (2005) as tertiary dominantly tidal sequences deposited in an inner-middle estuary. The section at Los Amigos, MD42B, is ∼12 m thick (Fig. 11) and was assigned to the Madre de Dios Formation (upper Miocene age) by Antoine et al. (2003). Our data, however, indicate that the sediments are fluvial in origin and belong to the lower of several Quaternary fluvial terraces that are easily recognized both in the field and on satellite images (Fig. 12). Radiocarbon dating on wood from the MD42B section yields an age of 25,305 ± 131 cal yrs, indicating that the section is late Pleistocene in age. The sample (OS-51223) was dated by AMS radiocarbon analysis at the National Ocean Sciences Accelerated Mass Spectrometer (NOSAMS) Facility, Woods Hole Oceanographic Institute (Rigsby et al., 2009). The resulting 14C dates were converted to calendar years before present (cal yrs BP) using the CalPal radiocarbon calibration program with the CalPal-2007-Hulu calibration curve. The sediments of the Los Amigos section, described by Antoine et al. (2003) as “excellent tidal facies,” are, instead, fluvial clayey and fine fl Fig. 6. (A) Pedogenic features in oodplain deposits (massive silty-clayey sediments) in sandy sediments that were deposited in an abandoned meander a BR 364 road cutting, location 9°37′17.25S/ 68°14′12W, area 3 of Fig. 1. Note that the floodplain paleosols are covered by “wetter” floodplain deposits with less pronounced (Fig. 11)(Rigsby et al., 2009). The sediments are characterized by pedogenetic features (above arrows). (B) Paleosol in a Rio Acre bank outcrop upstream sedimentary structures (ripples), grain sizes (sand and clay/silt), and from the profile described by Räsänen et al. (1995) at Amapa (area 2 of Fig. 1). The colors (gray to green/blue) that are similar to those found in outcrops extend from 10°02′1.74″S/67°52′29.09W for some hundreds of meters floodplain sediments of many rivers; for example those described in upstream on the left bank of the Rio Acre. The paleosol is found in a fine-sediment- the Mississippi River by Ray (1976) and Farrell (1987). Additionally, dominated sequence of floodplain deposits. Vertebrates as well as fresh-water bivalves were found in the floodplain deposits. the fossil mollusks reported by Antoine et al. (2003), two taxa of which were identified by F. Wesselingh at the genus level (Pisidium spec. indet., and Eupera spec. indet.), indicate a fresh-water 2.1. Sedimentology revisited depositional environment and are still living in modern Amazonia. Our interpretation of the Cerro Colorado section (Fig. 10) is also Several authors have suggested that the Solimões Formation different from that of Hovikoski et al. (2005). Hovikoski et al. (2005) sediments were deposited in tidal environments during a late interpreted this sequence as an upper Miocene tidal deposit. They Miocene sea-level highstand that resulted in marine transgressions performed a statistically forced analysis of laminated sediments and into the Amazon basin from the north (Caribbean Sea) or from the traces to demonstrate the influence of tides in the region. Our south (Paranáense Sea) (Räsänen et al., 1995). However, both the sedimentologic and geomorphologic analyses reveal that the se- existence of a seaway through western Amazonia during the late quence is, instead, a Quaternary fluvial sequence that rests uncon- Miocene and the existence of thousands of square kilometers of areas formably on upper Miocene sediments that contain the gomphoterid affected by tidal effect are difficult to support (Latrubesse et al., 2007). Amahuacatherium peruvium. The Cerro Colorado sequence is a 40+ m Moreover, the faunal composition and pollen content of the upper thick (Fig. 10) sequence of stacked fining-upward fluvial strata, Solimões Formation are inconsistent with a tidal and/or marine including, for example, a thick sandy cross bedded channel/point bar environment in southwestern Amazonia (Cozzuol 2006; Latrubesse et facies with mud rip-up clasts that fines upward to rippled and al., 2007; see below). The Acre River outcrops described by Räsänen et laminated overbank silt and clay. The unconformity outcrops at the al. (1995), for example, have been reinterpreted as fluvial deposits base of the section and can only be seen during a strong dry season (Westaway, 2006; Latrubesse et al., 2007). As discussed above, this (e.g., July of 2005, the dry season in the driest year on record in this sequence is similar to other fluvial sequences deposited in rivers part of the basin). carrying abundant suspended sediments. Additionally, in sequences The Cerro Colorado sequence is similar to those described in the exposed near the outcrops described by Räsänen et al. (1995) we Upper Jurua by Simpson and Paula-Couto, (1981) and by Latrubesse found fresh-water mollusks, fresh-water fishes, and terrestrial and Rancy (1998) as “pseudopuca type” sediments that means alluvial vertebrates — no evidence of marine or transitional sea/land sediments younger than the Late Miocene but that looks like older environments (Latrubesse et al., 2007; Fig. 5B). sediments. The Cerro Colorado sediments form a fluvial terrace of 106 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Fig. 7. Talisma, classic locality for the Acre fauna located on the right bank of the upper Purus River in southern Amazonas State, close to the border with Acre State (area 3 of Fig. 1, location 08°10′32.1S and 70°29′4.1W). The section, 8 m thick, is dominated by floodplain fine sediments, mainly silts and clays, with secondary gypsum and calcite veins in the lower part and manganese stains in the upper part. The deposits are characterized by massive bedding, bioturbation, and mottling. The exposed section measures 8 m and above this level the outcrops are covered by vegetation. Two fossiliferous levels, marked with asterisks, were found at 1.7 and 7 m above water level. ancient alluvia that is higher and older than the terrace described at channel are normally incomplete remains, with bias for the more Los Amigos (Figs. 10 and 12). The sediments of Cerro Colorado contain resilient structures. well-preserved sub-fossil logs that are of likely Quaternary age, Below we discuss chronology, vertebrate diversity, and the although radiocarbon dating yielded an indefinite age (N48 ka paleoenvironmental aspects of the fossil assemblage. Paleontological radiocarbon years BP, sample OS-56257, dated by AMS). Mega-lake information related to the geological localities described in Fig. 3, 6 theories have also been revived in recent years. Wesselingh et al. and 7 are detailed in Table 3. (2002) and Hoorn (2006b) postulated a giant paleolake, “Lake Pebas,” in western Amazonia during the middle to late Miocene. Our 2.2.1. Chronology interpretation, consistent with the interpretations of the Solimões Latrubesse (1992) and Latrubesse et al. (1997) suggested a late Formation in other parts of the basin (as outlined above) and in the Miocene to early Pliocene age (Huayquerian–Montehermosan land context of sedimentary architecture at both basin scale and mammal age) for the mammals from the Solimões Formation in Acre. depositional-system scale, is that of shifting rivers in aggradational However, more recent studies (Cozzuol 2006; Latrubesse et al., 2007) conditions, with floodplain facies and lateral accretion deposits, as allow us to assign the fossil assemblages to a late Miocene age well as local crevasse-splays. (Huayquerian mammal age, 9–6.5 Ma). Twenty genera found in the Acre region are also present in the Mesopotamian fauna of Argentina 2.2. Fossil fauna and Uruguay (Cozzuol, 2006; Latrubesse et al., 2007) and 10 species are also shared by both assemblages, including the rodents Pota- Land mammal assemblages continue to be a main important marchus murinus, Neoepiblema horridula and Phoberomys burmeisteri, method for correlation in continental environments of South America. as well as other amniote groups. Several shared taxa, such as the The fauna of Acre, which has been studied since the expeditions of toxodontid Grynodon and the rodents Cardiatherium and Phoberomys, Chandless in 1866, is one of the more complete and complex of the late indicate a good correlation with the upper member of the Urumaco Miocene of South America (Cozzuol, 2006; Latrubesse et al., 2007). fauna, from northern Venezuela (Falcon basin). Rancy (1985, 1991) and Webb and Rancy (1996) reviewed previous It is interesting to note that some taxa from western Acre found research in the region. Fossils in Acre have been collected from the near the Peru–Brazil border seem to indicate an older age (Campbell outcropping sections along the river banks (Figs. 6 and 7) and from the et al., 2000; R. Madden, pers. com., Sep. 2004), but this has not yet uppermost levels outcropping on the hills of the lowland dissected been confirmed. We think that an older fauna could be found on the plain at locations such as km 29 (Fig. 4), the Lula fossiliferous locality, west flank of the Divisor–Contamana Range in the oldest Cenozoic and along road BR 364, near the Caeté River. Most of the species listed units, such as the Ramon Formation, which outcrops there as a in Table 2 can be found in the Laboratory of Paleontology of the Federal consequence of local tectonic activity. University of Acre. These fossils were mainly found in situ and within the channel facies and the low energy/ floodplain–lacustrine facies 2.2.2. The vertebrate diversity described above. Because of the low energy of the environment, the The Acre area yields one of the most extensive and best known fossils in the floodplain–lacustrine–flooded areas assemblage are more faunal associations of northern South America. In general, there is a complete, better preserved, and frequently articulated. Those of the notable bias toward medium to large size animals, which may reflect a E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 107

Fig. 8. Examples of flood basins with internal delta fluvial systems such as are associated with large river systems in South America. (A) The Middle Paraná River floodplain at Santa Fe. Note the Delta Leyes coming from the main Paraná channel to the back lakes formed on the water saturated floodplain. 1 — Parana River channel, 2 — Meandering floodplain- branch channel, 3 — scroll dominated floodplain generated by the meandering branch, 4 — lake/pounded area, 5 — Leyes Delta, 6 — Setubal Lake; 7 — hindered drainage floodplain. (B) The Mato Grosso Pantanal flood basin. 1 — Uberaba Lake; 2 — lake and swampy plain related to channel that suffered sucessive avulsion; 3 — Cuiaba River floodplain/delta system; 4 — River Paraguai–Paiaguás floodplain/delta system; 5 — Paraguai–Corixo Grande mega fan. combination of preservation and collection bias. Smaller vertebrates, the emblematic “anaconda” in the Amazon basin. The extant species particularly mammals, are present, but in most cases they are not of Eunectes are adapted to a semiaquatic lifestyle, living in rivers, identified below family level (Campbell et al., 2000, 2006, and lakes, swamps, temporary pools, and flooded forests (Strimple, 1993). personal observation) or endemic, without biostratigraphic signifi- The fossils were related to Eunectus murinus, a genus that has cance (Czaplewski, 1996; Cozzuol et al., 2006; Kay and Cozzuol, 2006). currently spread throughout the forested areas of Bolivia, Brazil, As represented in the fossil record, the vertebrate groups clearly Colombia, Ecuador, Guiana, French, Guiana, Peru, Trinidad and show less diversity than those same groups show today in the region. Venezuela (Hsiou and Albono, 2009). Fig. 13A is a proportional representation of the diversity of mammals The first record of lizards of the Solimoes Formation also comes in the Acre record (restricted to those identified to at least genus from Talisma (Fig. 7). The remains were conferred to the extinct genus level); a list for the non-volant mammal taxa is given in Table 2. The Paradracaena ( Hsiou et al., 2009). The extant genus Dracaena is a absence of conspicuous North America immigrant taxa indicates that large, semi-aquatic teiid, represented by two species, that inhabit the Acre assemblage precedes the main flow of the Great American swamps and wetland environments related to large rivers and dense Biotic Interchange (GABI). However, the recent finding of a probos- vegetation (Krause, 1985) along the Amazonia Basin, Mato Grosso cidean (Campbell et al., 2000) could suggests that the interchange had State and Paraguay (Vanzolini and Valencia, 1965; Ávila-Pires, 1995). already started by this time. The habits of Paradracaena would have been similar to the extant There is no evidence of a marine environment in the fossil record. species (Hsiou et al., 2009). Bony and cartilaginous bony fishes, The most abundant specimens, in both diversity and number, are reptiles, and river dolphins are indicative of the presence of a large crocodilians and aquatic turtles. Crocodiles are represented by four flooded basin dominated by shallows lakes and swampy areas crossed families and have a large diversity both in number of taxa and by and closely associated with dynamic alluvial belts. Many of the fish morpho-ecological types, much larger than the present-day diversity taxa in the fossil record are still living in the region today, such as in the region and in any other time and locality in the world (Cozzuol, species of the genera Lepidosiren, Arapaima, Colossoma, Phractocepha- 2006). The river turtles of the family Pelomedusidae are endemic to lus and Potamotrygon, among others. South America and good paleoenvironmental indicators. They Among the mammals, which are the main basis for the South indicate stable water bodies with abundant vegetation and tropical American Tertiary stratigraphic scheme, rodents are quite diverse. to subtropical climate, in addition to continental conditions. The Thirteen taxa (identified at least to generic level) have been genus Eunectus (serpentes,boidae) had been recently recorded in the represented in the Acre study. They indicate a diverse environment Talisma locality (Fig. 7)(Hsiou and Albono, 2009). This is the first and include species that based on their inferred diet (see below), lived record of snakes in the Solimoes formation and the first fossil record of in areas close to water bodies as well as those that lived in open grass 108 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Fig. 9. Studied wells: (A) 1AS-32-AM, (B) 1AS-27 AM, (C) 1AS-19-AM. See location in Fig. 1. savannas. Eleven nominal taxa of notoungulates of the family been found in nearby Peruvian localities (Campbell et al, 2000). In any Toxodontidae have been identified to at least the genus level in the case, the presence of toxodontids indicates open environments. For Acre record (Cozzuol, 2006). This diversity seems to be too high, and some toxodontids, a semiaquatic lifestyle has been suggested, but this some preliminary data suggest this number should be reduced to no is difficult to assess with the available material (mainly crania and more than six genera (Ribeiro, 2005, Ribeiro et al., 2005) (see Tables 2 mandibles). Litopterns are represented by one genus of the family and 7). However, other taxa that have not yet been described have Macrauquenidae and one protherothere, probably Protherotherium

Fig. 10. Cerro Colorado sequence, a 40+ m high section showing stacked fining-upward fluvial sequences younger than late Miocene and probably Quaternary in age, as suggested by the morpho-stratigraphic regional position (see Fig. 11) and the presence of sub-fossil wood. The unconformity with Tertiary sediments outcrops at the base of the section and can be seen only during a very dry season. Photos show the uppermost floodplain point bar sequence (left) and rip-up clasts in the crossbedded point bar sequence at approximately 7 m from the base of the section (right). E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 109 110 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Fig. 11. Sediments of the Late Pleistocene fluvial terrace of the Los Amigos section (MD42B) in the Madre de Dios fluvial basin, Peruvian Amazon. Radiocarbon dating on wood from the section indicates that the deposits are late Pleistocene in age. Sample locations are indicated by asterisks; symbols are explained in the graphic section show in Fig. 9.

(Frailey, 1986; Bergqvist et al., 1998; Cozzuol, 2006). This is a water like the large present day rodent capivara (Pascual and relatively low diversity if compared with other South America Bondesio, 1985; Latrubesse et al, 1997). localities, even the more similar associations of Mesopotamian and The presence of primates Platyrrhini, which rarely descend to the Laventan (Cozzuol, 2006). ground, may indicate the existence of gallery forests along the rivers Remains of an indeterminate Astrapotheria (Paula Couto, 1982) (Kay and Frailey, 1993; Latrubesse et al., 1997; Kay and Cozzuol, and the gomphothere Amahuacatherium peruvium (Campbell et al., 2006). 2000) may be interpreted as indication of open areas with nearby forested environments. Xenarthra, amongst which the ground-sloths 2.2.3. Summary of diet habits of mammals as environmental indication were the dominant form, represents the third largest group of taxa in The diet of the mammals from Acre was inferred from craniodental the Acre record, with six taxa identified to genus or species level, morphology and analogy with living representatives or similarities to representing three families. Giant armadilos (Pampatheridae) and unrelated living taxa. A summary of these inferences for terrestrial Glyptodonts are also present; they are much less diverse than in the non-volant mammals can be found in Table 2 and Fig. 13B. Mesopotamian fauna, but not much different from that at La Venta Grazers are the most common mammalian forms. This category (Laventan fauna). includes all the Notoungulata, several large rodents, as well as Rodents are also very extremely diverse (Boquentin-Villanueva, glyptodonts and pampatheres (Xenarthra). These taxa indicate open 1998) with 10 genera and 14 species recorded and additional new grass covered environments. The significant number of browsers rodent taxa from the UFAC collections are being described. Rodents (ground sloths, litopterns, and gomphotheres) and fruit/leaf eaters such as Kyutherium (Cardiatheriinae) inhabited areas near bodies of (monkeys and smaller rodents) suggests forest niches. These niches E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 111

Fig. 12. (A) Digital model of terrain obtained from SRTM (Shuttle Radar Topographic Mission) images, processed in ENVI-GIS and (B) Landsat 5 TM image. OAL = Old Quaternary Alluvia; PDQL = Piedmont Quaternary Alluvia; LT/FP = Late Pleistocene-Holocene sediments (lower Terrace and present floodplain). 1 = Cerro Colorado section (12°34′00.45″S; 70°06′18.58″W); 2 = Los Amigos (MD42 B) section (12°33′26.29″S; 70°05′29.45″W). were probably, at least in part, related to water bodies and occurred as In summary, the vertebrate record from Acre is compatible with an river-edge gallery forests or lake-margin forests. This category of environmental model that includes grasslands and river swamp, and mammals also includes the litopters Astrapotheria and Protherorthi- lake-side gallery forests which were subject to a fluctuating water dae, which are not included in the list because they have not been level in a seasonally flooded tropical to subtropical wet-dry climate. identified to genus or species level. The paucity of non-volant mammals with an “animalivourous diet 2.2.4. Freshwater mollusks and ostracods style (only one marsupial), although certainly related to sampling The Solimões Formation is rich in fresh-water mollusks. Several problems, is not completely unexpected. authors have proposed that occasional early to middle Miocene Animalivorous mammals are also quite rare in La Venta, and the marine incursions from the Caribbean reached as far as 20S(Hoorn, role of predators may have been taken by the highly diverse 1993, Vonhof et al., 1998). However, non-marine mollusks have been crocodiles. Carnivorous mammals were not found. The only repre- recorded in the Solimões Formation, including the key Amapa sentative is the opossum Didelphis solimoensis (Cozzuol et al. 2006), outcrops that Räsänen et al. (1995) used to propose the late Miocene which represents the oldest species of this family. This animal exhibits marine ingression across southwestern Amazonia. The outcrops dental features related more to a frugivory than an animalivorous diet. extend from 10°02′1.74″S, 67°52′29.09W to some hundreds of meters In any case, this species hardly plays an important role as a upstream on the left banks of the Acre River (Fig. 5B). The sequences carnivorous mammal. The absence of specialized carnivorous marsu- incude paleosols in a sequence of floodplain deposits that are pials typical of South American faunas, such as the Borhyaenidae and dominated by fine grained sediment. Vertebrates and freshwater Thylacosmilidae families founded in La Venta, should be interpreted bivalves, such as Pachydon sp., Castalia sp., Prisodon sp., Diplodon sp., mainly as a deficiency of the record more than in their absence. and Mycetopoda sp. (material identified by María Inês Feijó Ramos, 112 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Table 2 found only freshwater mollusks in outcrops of the Solimões Late Miocene non-volant mammals of the Solimões Formation in the Acre Region, with Formation, including some of the localities described by Hoorn hypothesized macroniche specialization. Dietary categories: G = grass, stems and (1994a). Wesselingh et al. (2006), studying several outcrops of the leaves (grazers); FL = fruit with some leaves; L = leaves (browsers); IF = primarily insects with some fruits; Ve = primarily vertebrate prey. Locomotor or substrate Solimões Formation in Acre and Amazonas States, concluded that the preferences: T = terrestrial and fossorial; SA = semiaquatic; AT = arboreal and Solimões fauna (including unionids, ampullariids, and sphaeriids) is terrestrial (scansorial); A = arboreal. dominated by pearly fresh-water mussel species similar to those that fl Rodentia Diet Substrate today inhabit oodplain lakes as well as small and large rivers of the Amazon basin. The localities studied by Wesselingh et al. (2006) Neoepiblemidae ′ ″ ′ ″ Neoepiblema horridula GTinclude Cachoeira Bandeiras (10°56 22 S and 69°20 37 W), a locality, Neoepiblema ambrosetianus GTwell studied by our team and by vertebrate paleontologists of the Phoberomys minima GTFederal University of Acre, that contains abundant fossil vertebrates of Phoberomys burmeisteiri GTlate Miocene age (Huayquerian–Mesopotamian). Eight ostracod Phomeromys bordasi GT species found in outcrops of the Tarauacá River (Torre da Lua locality) Hydrochoeridae fl Kiyutherium orientalis GSAand from oodplain deposits of the Solimões Formation near the Dinomydae border between Amazonas and Acre States indicate freshwater Potamarchus sigmodon FL T environments in this area (Feijo Ramos, 2006). Six species are typical Potamarchus murinus FL T of freshwater environments, and Feijo Ramos (2006) argued that the Tetrastylus sp G T Gyriabrus sp G T two Cyprideis species, considered an indicator of brackish conditions Eumegamys paranensis GTof the Pebas fauna by other workers, represent freshwater settings Telicomys amazonensis GTequally well. Diversified and abundant non-marine coeval calcareous Simplimus sp FL AT microfauna including the presence of foraminifera in continental Dasyproctydae environments had been studied in Brazil (Purper and Pinto, 1983; “Scleromys” colombianus FL T Xenarthra Purper and de Ornellas, 1991. Results by Muñoz-Torres et al. (2006) Tardigrada indicate that, in the Upper Amazon basin, the genus Cyprideis suffered Orophodontidae a strong evolutionary radiation in continental lacustrine deposits Octodontobradys puruensis LTduring the Miocene. The genus Cypridus, occurs presently in African Megalonychidae Pliomorphus sp L T fresh water lakes (Feijo Ramos, 2006). This genus was also common in Mylodontidae lacustrine sediments of the Sahara during the Quaternary and Tertiary Urumacotherium sp L T lake sediments in Spain. Cypridus is associated in the Ebro Duero and Acretherium campbelli LTBazan basins of Spain, along with other continental marine-like Pseudoprepotherium sp L T organisms, such as foraminifera (including Ammonia beccarii, Elphi- Ranculus sp L T Cingulata dids and other rotalids, trochamminids, miiiolids, etc.), ostracodes Pampatheridae (i.e. Cyprideis torosa and Loxoconcha) and molluscs (“Cardium”, Abra, Kraglievichia sp L–GTPotarnides, etc.) (Anadón, 1992). Glyptodontidae Plohophorus sp L–GT Paraglyptodon sp L–GT2.3. Palynology Asterotema sp L–GT Notoungulata 2.3.1. Biostratigraphic data Toxodontidae A great advantage of palynology is that vegetation can be studied Abrothrodon pricei GTby analyzing sediment cores, thus revealing more information than Gyrinodon quasus GT Trigonodops lopesi GTprovided by the outcrops alone. As discussed by Latrubesse et al. Trigodon sp G T (2007), the Miocene biozones proposed in western Amazonas by Plesiotoxodon amazonensis GTHoorn (1993) should be used with some caution because the time Toxodontherium listai GTranges of the main species used as biostratigraphic markers appear to Neotrigodon utoquineae GT fi Neotoxodon pascuali GToverlap zone boundaries. Therefore as de ned, the zones offer only a Mesenodon juruaensis GTrough temporal calibration within the middle to late Miocene. Late Mesothoxodon pricei GTMiocene and Pliocene markers appear infrequently in the palynolog- Minitoxodon acreensis GTical associations, because of the low frequency of those markers in the Litopterna total assemblage. On the other hand, the high number of under Macrauchenidae Culinia sp L T described species founded in Amazonian Neogene samples reveals the Proboscidea great potential the study of the palynology of Amazonia has for the Gomphotheriidae improvement of the current palynological biostratigraphic resolution Amahucatherium peruvium LT(Silva-Caminha et al., 2010). Primates Palynological samples from Solimões Formation were analyzed from Cebidae Acrecebus frailey FL A wells 1AS-32-AM, 1AS-27-AM and 1AS-19-AM and from several outcrops Stirtonia sp L A along the Upper Acre River (Fig. 1). A total of 54 core samples were Atelidae analyzed: 13 samples from core 1AS-32-AM (4°32′38″S/71°24′24″W), Solimoea acrensis FL A located on the Curuçá River, Amazonas, 33 samples from the 1AS-27-AM Marsupialia ′ ′ Didelphidae (4°17 S/67°55 W) drilled to the depth of 402.5 m at Jutaí River and Didelphis sp IF-Ve AT 8 samples from well 1AS-19-AM (04°33′S/69°10′W), drilled to a depth of 255.7 m near the Jandiatuba River, São Paulo de Olivença city, Amazonas (Figs. 1 and 9). Museu Paraense Emilio Goeldi, Belém, Brazil), occur in these floodplain Additionally, outcrop samples were collected from fossiliferous deposits. sites on the river banks in southwestern Amazonia, including Patos Wesselingh et al. (2002) reported freshwater mollusks in the (10°55′00″S/69°55′00″W), Barranco da Elizete (10˚56′25″S/69˚45′44″W), Pebas Formation and, in a more recent study, Kaandorp et al. (2005) Murici (57′24″S/69˚44′44″W), and Niterói (10°08′14″S/67°48′46″W) on E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 113

Table 3 Paleontological description of some main localities of the Solimões Formation (Late Miocene).

Locality and geological Paleontological record Comments description

Patos Proterotheriidae and small rodents (personal observation), among many This locality is also referred in the literature as LACM 4611 or Acre 6 For geological other taxa, are recorded here. A few normally well-preserved specimens (Frailey, 1986; Campbell et al., 2000) Cozzuol, in press), (Cozzuol et al., description see were found including a medium-sized rodent mandible (family in press), (Bergqvist et al. (1998)), (Frailey, 1986; Kay and Frailey, Fig. 6 Inomyidae), a partial,skull of a crocodile, and compressed seeds and 1993), Palynological samples are in agreement with the Late Miocene marsupials bats (Czaplewski (1996); and personal observations by ther age assignment (Silva, 2004; see palinology section of this paper. authors). Km 29 Two levels with “in situ” fossil vertebrates are identified at 1.85 and 8 m Hovikoski et al. (2003, 2005) report trace fossils and stratigraphic For geological from the base of the section. The specimens found in these levels are well- structures from this locality, which they interpret as an indicators description see preserved, some of them are partially articulated, and having medium to of an estuarine, marine-influenced (freshwatermesohaline) Fig. 3. large size. A gavialiid skull (probably Hesperogavialis sp.), lungfishes environment. However, all the evidence provided by the fossil (genus Lepidosiren), other fish remains, and well-preserved freshwater vertebrates and invertebrate assemblages indicate a definitive crabs were recovered at the lowest level. The upper level yielded a skull of freshwater environment for the entire section. Purusaurus brasiliensis (Alligatoridae), a Crocodilidae, a Gavialidae, actinopterigian fishes (mostly Siluriformes), terrestrial mammal teeth, crocodile teeth, freshwater crabs, and bivalve mollusks. The specimens from the lower level seem better preserved than those from the upper level. Sparse and fragmentary vertebrate remains (mostly crocodile teeth, fish spines, and vertebrae) are also found in the section.

the Acre River, Acre State (Fig. 1). These results were compared and correlated with those from well samples.

2.3.1.1. Well IAS-32-AM. The 132 m-deep well 1AS-32-AM (located ∼54 km southwest of well 1AS-4a-AM that was studied by Hoorn, 1993; see Fig. 1) is divided in recent sediments, from 0 to 13 m and Solimões Formation from 13 to 133 m and is composed mainly by silt, clay and few levels of lignite. (Fig. 9A). The Solimoes Formation shows high abundance of both Grimsdalea magnaclavata and Crassoretitriletes vanaadshoovenii, and was inter- preted by Latrubesse et al. (2007) as having middle to late Miocene age, zone of Grimsdalea according to the zonation of Lorente (1986). The base of the Grimsdalea Zone is defined by the first occurrence of Grimsdalea magnaclavata and the top is defined by the first occurrence of Echitricolporites spinosus, Fenestrites spinosus or F. longispinosus (Lorente 1986). However, we have reanalyzed samples from this well and have found Psilatricolporites caribbiensis to 106 m, 120 m and 126 m. P. caribbiensis is key specie because its oldest occurrence is recorded in the zone of Fenestrites longispinosus of Lorente's zonation (1986) in Venezuela ranging from the Late Miocene to the early Pliocene. Therefore, the age of the 1A-32-AM is assigned to the late Miocene and possibly can reach the early Pliocene. Because of the uncertainties in the temporal definition of palynological biochron, however, the extension of the record to the early Pliocene needs to be taken with caution and studied in future with more detail.

2.3.1.2. Well IAS-19-AM. The 218 m-deep well 1AS-19-AM is com- posed of very fine sand, clay and some fine levels of lignite (Fig. 9B). Variations in gamma-rays enhance the presence of finer dominated sequences from 11 to 172 m, and an increasing of sandy sedimentary bodies from 172 m to the base. The well 1AS-19-AM was considers as belonging to the zone of F. longispinosus (sensu Lorente 1986), because of the presence of fenestrate pollen grains and P. caribbiensis to 109 m and 218 m depth.

2.3.1.3. Well 1AS-27-AM. The 400.5 m-deep well 1AS-27-AM is characterized by the dominant presence of clay and silt levels to the top and sandstone bodies that become more abundant toward the base (Fig. 9C). The well 1AS-27-AM was also related to the zone of F. longispinosus of Lorente 1986 because the record of P. caribbiensis to 216 m and 221 m depth.

Fig. 13. (A) Mammalian diversity. (B) Type of diet of the mammals recorded in the 2.3.1.4. Outcrops samples. Outcrop samples from river bank localities Solimões Formation, Acre. along the Acre River, such as Patos, Barranco da Elizete, Murici and 114 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Niterói (Table 4), yield Asteraceae pollen, suggesting a late Miocene 3 m above the water level, from clays rich in organic material, age. including lignite. The locality is characterized by abundant Grimsdalea Thus, the palynological data pollen could agree with the late magnaclavata and absence of Asteraceae pollen. Miocene age indicated by the fossil vertebrate content. Localities and The following taxa are recorded: assemblages are summarized below. Grimsdalea magnaclavata (28%), Deltoidospora adriennis (13%), 2.3.1.4.1. Patos, Upper Acre River (Fig. 6). At this location, pollen Verrucatosporites usmensis (9%), Psilatriletes sp. (8%), Monoporopolle- was collected from the base of an intraformational conglomerate nites annulatus (7%) and Perisyncolpites pokornyi (6%). Verrutriletes sp. consisting of clay pebbles and silt, about 0.5 m above the water level. Psilatriletes sp, and M. grandiosus each constituted 3%. Less abundant This conglomerate is rich in fossil vertebrates of Huayquerian– were Mauritiidites franciscoi, Azolla sp. and Bombacacidites sp. 3, Mesopotamian SALMA equivalent age. Retitricolporites sp, Psilatricolporites minimus, Retimonocolpites sp., The palynological assemblage is composed mainly of Monopor- Proxapertites tertiaria. opollenites annulatus (69%), followed by Echitricolporites spinosus (5%), 2.3.1.4.4. Niterói, Lower Acre. The Niterói site is located on the right Echitriletes sp. (4%), Fenestrites sp. and Verrumonoletes sp. (with 3% bank of the Lower Acre River, close to the town of Senador Guiomard, each), and Verrutriletes sp. and Psilatriletes sp. (with 2% each). Rare Acre State. Lignite and leaves are common in this site. Palynological occurrences of Magnastriatites grandiosus, Azolla sp., Chomotriletes samples were collected about 1 m above the water level. Conglom- minor, Cicatricosisporites sp., Psilatricolporites sp., Psilatriletes sp2., erates were not seen, but local, small intraconglomerate lenses have Psilatriletes sp, Retimonocolpites sp., S. catatumbus and Echitriletes been described at this locality (Latrubesse et al., 1997). This is an muellerii, Bombacacidites nacimientoensis, Polypodiaceiosporites sp., important locality with fossil vertebrates of Huayquerian–Mesopotamian Polyadopollenites sp., Perisyncolporites pokornyi, Podocarpidites sp. and age. Matonisporites sp. The assemblage found in Niterói includes Monoporopollenites In this same locality were collected “in situ” seeds of Myrtaceae annulatus (15%), Crototricolpites annemariae (8%), Corsinipollenites and Euphorbiaceae (genus Piranhea), indicating a typical tropical oculusnoctis (6%), Psilatriletes sp., Psilatricolpites sp., and Retitricolpor- floodplain (“várzea”) environment. ites sp. (5% each). Cicatricosisporites sp. (4%) and Illexpollenites sp., 2.3.1.4.2. Barranco da Elizete, Upper Acre River. This locality is on the Echitricolporites maristellae, Verrumonoletes sp., Retimonocolpites sp. left bank of the Acre River and the samples were taken from a dark and Striatricolpites catatumbus (3% each), Mauritiidites franciscoi, clay lens at the water level. The assemblage contains an abundance of Verrutriletes sp., Retitriletes sp., Retitricolporites sp., Margotricolpites spores of Psilatriletes sp. (15%), Verrutriletes sp. (12%), Verrumonoletes sp., K. waterbolkii and Polypodiaceoisporites sp., Echiperiporites sp. (2% sp. (10%), Concavisporites sp. (9%) and Psilatriletes sp. (7%). Pollen each), and Azolla sp., Bombacacidites sp., Echitriletes muellerii, P. grains consisted of Echitricolporites spinosus (8%), Echitricolporites herngrenii, P. tertiaria, Psilaperiporites sp., Psilatriletes sp, Psilatricolpor- maristellae (6%), Echiperiporites sp. (5%), and Bacutriletes sp. (4%). Also ites sp., Echitricolporites spinosus (1%). Indeterminate palynomorphs found in low frequencies (1%) were Cicatricosisporites sp., Echitriletes constituted 10% of the sample. sp., Matonisporites sp., Kuylisporites waterbolkii, Fenestrites sp., Illexpollenites sp. Polypodiaceoisporites sp., Retitricolpites sp., Echipolle- 2.3.2. Paleoenvironmental considerations nites sp. and E. estelae. Indeterminate types constituted 5% of the Hoorn (1993) proposed that marine incursions had occurred in sample. part of Amazonia and characterized coastal environments based on 2.3.1.4.3. Murici, Upper Acre River. This locality is on the right bank the presence of Rhizophora (Zonocostites ramonae) and Acrostichum of the Acre River, near the town of Assis Brasil, Acre State. No (Deltoidospora adriennis) in some intervals of well IAS-4a-AM, megafossils were found here. The samples were collected from about claiming that the sea-level rise took place during the early to middle/late Miocene, in agreement with the eustatic sea-level stages described by Haq et al. (1987). However, it is important to note that Table 4 no marine or coastal environmental indicators were found in either of Frequency of the main pollen types founds in surface samples determined by the the core samples (wells IAS-32-AM and IAS-19-AM, IAS 27-AM) or in absolute amount on a total of 300. the outcropping sections in Patos, Niteroi, and Barranco de Elizete. Types Sites Our late Miocene pollen data from Acre outcrop samples contain high abundance of Monoporopollenites annulatus (Poaceae), which Barranco da Elizete Patos Niterói constituted nearly 69% and 15% of the pollen at Patos and Niterói, Grimsdalea magnaclavata –––respectively (Table 4). In Patos, the presence of Piranhea seeds and the Crototricolpites annemarie ––(23) abundance of Poaceae pollen (Monoporopollenites annulatus) could be Proxapertites tertiaria ––(3) fl Monoporopollenites annulatus – (207) (44) indicative of a typical oodplain environment. In Niteroi, the Perisyncolporites pokornyi – (1) (1) palynological records are characterized by the abundance of spores Echitricolporites spinosus (10) (16) (3) and Corsinipollenites oculusnoctis, also suggesting the presence of ––– Deltoidospora. adriennis swamps. Magnastriatites grandiosus – (2) – Mauritidiites franciscoi ––(5) Pollen composition from Acre localities suggests an environment Verrutriletes sp. (12) (6) (5) characterized by grasslands, floating meadows, and gallery forests Psilatriletes sp. (19) (7) (15) along rivers, swamps, and shallow lakes (Table 5), as indicated by our Azolla sp. – (2) (4) vertebrate fossil records. The levels sampled for pollen are also rich in Retitricolpites lorentae ––(2) fossil vertebrates. Bombacacidites sp. ––(4) Chomotriletes minor – (2) – In well 1AS-19-AM, spores were abundant in all sections, as well as Echitricolpites maristellae (7) – (10) by pollen of Poaceae, Arecaceae (palms), and Euphorbiaceae. This Illexpollenites sp. (3) – (10) assemblage also suggests an environment with swamps and forests, Verrumonoletes sp. (15) (7) (10) as indicated in previous sections from Acre and Amazonas. Cicatricosisporites sp. (2) (2) (11) Fenestrites sp. (2) (9) – About 30 angiosperm families are represented in both wells and Kuylisporites waterbolkii (2) – (5) outcrops, suggesting that the Amazonia diversity have been present Echitriletes muelleri – (3) (3) since that time. When comparing the Late Miocene with Quaternary Corsinipollenites oculusnoctis ––(18) palynofloras (Absy, 1979) the number of Quaternary angiosperm Striatricolpites catatumbus – (1) (10) families is higher (∼37 families). E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 115

Table 5 Environment and climate inferences related to the pollen record.

Groups Taxa Affinities Habit/environment Climate Reference Localities

Pteridophyta Azolla Salviniaceae Aquatic Hoorn (1993) W/O Cyathidites Cyatheaceae Tree-like Tropical and subtropical Antonioli, 2001 W/O Cicatricosisporites Schizaeaceae Herb Tropical and subtropical Antonioli, 2001 W Chomotriletes Schizaeaceae Herb Tropical and subtropical Antonioli, 2001 O Crassoretitriletes Schizaeaceae Swampy or costal Hoorn (1993) W Deltoidospora adriennis Cyatheaceae/Gleicheniaceae Freshwater Tropical and subtropical Antonioli, 2001 W/O Collinson, 2002 Gleicheniaceae Gleichenidites Herb Tropical and subtropical Antonioli, 2001 W/O Magnastriatites Parkeriaceae Costal/alluvial Hoorn (1993) W/O Matonisporites Matoniaceae Herb Tropical humid Antonioli, 2001 W Verrucosisporites Osmudaceae Herb Tropical humid and Antonioli, 2001 W/O temperate Verrucosisporites usmensis Polypodiaceae Lowland and montane forests Hoorn (1993) W Angiosperms B. baculatus Bombacaceae/ Pachira Lowlands along rivers Hoorn (1993) W Corsinipollenites oculusnoctis Onagraceae Swampy Hoorn (1993) W/O Crototricolpites annemariae Euphorbiaceae Herb W Echiperiporites estelae Malvaceae Costal plain Hoorn (1993) W/O Echitricolporites maristellae Bombacaceae /Malvaceae W Grimsdalea magnaclavata Arecaceae (?) Germeraad et al. W/O 1968 Mauritiidites franciscoi Arecaceae Swampy Hoorn (1993) W/O Monoporopollenites annulatus Poaceae Open or aquatic environment Tropical dry and Antonioli, 2001 W/O temperate Mutimarginites vanderhammeni Acanthaceae Shrubs / Herb Germeraad et al. W 1968 Perisyncolpites pokornyi Malphigiaceae Costal plain/Forest Hoorn (1993)/Jaramillo & W/O Dilcher, 2001 Proxapertites tertiaria Anonnaceae Trees/Herb Tropical dry Lorente (1986) W Retitricolpites lorentae Bombacaceae Trees Hoorn (1993) W

Some families/genera, found in the cores 1AS-27-AM and 1AS-19- palynology are in agreement with this kind of sedimentary model, AM, are also found presently in Amazonia and are recorded in this with abundant vegetation, grasslands and gallery forests along rivers, study for the first time in the Tertiary record of South America. This is as well as floating meadows in swamps and shallow flood basin lakes the case of Sapium (Euphorbiaceae), Byttneria (Malvaceae), Polygo- that sustained many permanent water bodies (Figs. 8 and 14). We num (Polygonaceae), Rauvolfia e Geissospermum (Apocynaceae), consider that the climate was tropical to subtropical with a dry season Loranthaceae, Psychotria (Rubiaceae), Caryophyllaceae and Gom- that produced stage variability in the fluvial systems, allowed lateral/ phrena (Amaranthaceae). Our data indicate that remarkable specia- horizontal ecological connectivity and that favored avulsion process- tion happened in the tropics during the Upper Miocene. Our proxy es. Some lakes were slightly saline, as supported by microfossils data are in agreement with the evolutionary rate analysis of studies, but aridity was weak and insufficient to produce calcareous Pennington et al. (2004), which suggested the diversification of the soils (calcretes) or evaporates. tropical floras during the Miocene and Pliocene. The majority of plant families found in the Upper Miocene are still 3.1. Correlation of the Solimões Formation with others lithostratigraphic present in the Amazon forest and other areas of tropical South units of South America America today while rare families/genera have became extinct since that time. The Acre faunal assemblage mainly pre-dates the Great American Biotic Interchange (GABI), as indicated by the absence of immigrant 3. Amazonia during the Late Miocene mammals such as the Sigmodontine rodents. Nevertheless, some elements suggest that a low rate of exchange The record of late Miocene sediments of the Solimões Formation had already been initiated at this time between the Americas indicates a continental origin. Fluvial sequences are characterized by (Campbell et al., 2000, 2001, 2006; Cione et al., 2001; Cozzuol, 2006). the predominance of multistoried sandstones and overbank mud- A total of 224 genera (we avoided taxa only identified at family or stones, paleosols, and an abundant fossil content. The geological higher levels) are present in the Acrean, Urumaco and Mesopotamian setting was a subsiding foreland basin extending eastward like a huge faunal assemblages, 125 genera in the Mesopotamian, 51 in the Acre, flat plain and dominated by avulsive rivers, lakes, swamps, internal and 23 in the Urumaco (Linares, 2004; Sanchez-Villagra, 2006; flood basin deltas, and some drier floodplain surfaces producing Sanchez-Villagra and Aguilera, 2006). The disparity between the paleosols. numbers of taxa in each assemblage could be due to differences in Both megafan systems and hyper-avulsive rivers coming from the sampling intensity. This is confirmed by the number of new taxa Andes generating a mosaic of floodplains and entering and crossing described for the Acre region in just the last few years (see Campbell flood basins and less flooded lowland areas could be appropriate et al., 2000, 2006; Cozzuol et al., 2006; Kay and Cozzuol, 2006) and by models for this time period (Fig. 14). Latrubesse et al. (2007) many specimens still unstudied in the collections at the Laboratory of suggested some analogies between the Solimões Formation environ- Paleontology of the Federal University of Acre. Tables 6 and 7 present ments of deposition and the Quaternary megafan systems of the the similarity coefficients and the number of shared genera and Chaco, the Pantanal wetlands, and the megadepositional system of species between the different regions. sedimentation of the Gangetic plain in India, where megafans are Cozzuol (2006) and Latrubesse et al. (2007) used these faunal typical (Siwalik sediments and Quaternary units). The paleoecological assemblages to correlate the uppermost levels of the Solimões interpretation of the fossil record of late Miocene vertebrates and Formation in southwestern Amazonia with the Upper member of 116 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Fig. 14. Flood basin schematic environmental model for the Solimões Formation during the Late Miocene. 1) Hyper-avulsive channels as recorded today in the Beni plain and Gangetic plain; 2) Megafans systems formed by avulsive channel belts coming from the Andes or highlands, such as the Chaco and Pantanal megafans of today. Abandoned alluvial belts become reused by local fluvial systems or occupied by swamps. 3) Lakes in the flood basin with lacustrine deltas; 4) Backswamps/lacustrine areas with deltaic crevasse-splay deposits. 5) Drier floodplain with soil formation on fine grained (muddy) deposits. The environment was characterized by a complex mosaic of widespread swampy areas, herbaceous vegetation, areas with gallery forests along fluvial belts (active and abandoned), and forest (“terra firme” forest) in the drier floodplain. the Urumaco formation in Venezuela, to the north, and the lowermost marine regression is marked by the deposition of the fluvial sediments levels of the Ituzaingó Formation and the Kiyu Formation, to the of the Ituzaingó and Kiyu formations. The lowermost conglomeratic south, in the La Plata basin (Fig. 15). In the latter area, a late Miocene sediments of the Ituzaingó Formation and the Kiyu Formation are rich in late Miocene continental mammals (Mesopotamian fauna). Table 6 Similarity coefficients between the Late Miocene northern South American faunal assemblages (Solimoes Formation–Acre fauna; Urumaco Formation–Venezuela, Ituzaingo 3.2. The Amazon basin: evolutionary approach Formation,–Mesopotamian fauna,–Argentina, La Venta fauna–Honda Group–Colombia) : A) Dice, B) Jaccard, C) Simpson. We will present a tentative model for the evolution of the Amazon A) Laventan Mesopotamian Acre Urumaco basin during the late Miocene. The relationships between the Solimões Formation and the Laventan 1 0.069307 0.20968 0.095238 Mesopotamian 1 0.23864 0.21656 tectonic evolution of the northern Andes during the late Miocene Acre 1 0.50633 are difficult to explain in light of the available information. Hoorn Urumaco 1 (1993, 1994a) assigns the outcropping sediments of the Colombian Amazonia (Mariñame Formation) to the early Miocene; it appears, B) Laventan Mesopotamian Acre Urumaco therefore, that late Miocene strata were not deposited in Colombian Laventan 1 0.035897 0.11712 0.05 Amazonia, where the Araraquara Formation (Paleozoic) also outcrops. Mesopotamian 1 0.13548 0.12143 If Hoorn's interpretation is correct, evidently there was no specific Acre 1 0.33898 – Urumaco 1 sedimentary response in the Marañon Solimões basin to the tectonic activity of the northern Andes during the late Miocene. However, in C) Laventan Mesopotamian Acre Urumaco the Putumayo basin, in Colombian Amazonia, there is about 1000 m of Laventan 1 0.093333 0.26531 0.16667 the Ospina formation, which has not yet been studied in detail and is Mesopotamian 1 0.42857 0.56667 probably late Miocene in age. Acre 1 0.66667 In contrast, in the central and northern Colombia llanos, the late Urumaco 1 Miocene is represented by thousands of meters of the continental and E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 117

Table 7 Table 7 (continued) Recorded genera of the South American faunas of the Late Miocene Solimões Formation (Acre), Urumaco Formation (Venezuela), Ituzaingo Formation (Mesopotamian fauna- Argentina) and the middle Miocene La Venta fauna (Honda Group, Colombia) (only amniotes are considered). Gray cells indicate the record of genera for each unit.

(continued on next page) 118 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Table 7 (continued) Table 7 (continued)

clastic sediments of the Guayabo formation (Cooper et al. 1995, Bayona, et al., 2008); they represent the eastward advance of a coarse clastic wedge that accumulated in continental fluvial and alluvial systems (Bayona et al., 2008). Tectonic control of the Amazon basin during the late Miocene seems strongly related to the central Andes, between ∼2° and 15°S, rather than to the northern Andes (Fig. 15). Late Miocene sedimen- tation in the Amazon basin extended eastward into the intracratonic Amazon basin. The isopach map (Fig. 1) of the Solimões Formation shows a well-developed trend with increasing thickness in south- western Brazilian Amazonia and thinning in the direction of the Brazil–Peruvian border, typical of a foreland basin. The Iquitos structural high separates the region into two interconnected late Miocene sub-basins (Fig. 1). Note that the Iquitos arch, as described by the Brazilian geologists of CPRM, is located eastern of Iquitos and extend from the Brazilian–Colombian border (Tabatinga) to the southeast, inland from the Amazon basins. The Eastern Cordillera of the central Andes was being uplifted and eroded between ∼25 and 10 Ma (early to middle Miocene). This uplift permitted the influx of sediments to the Amazon foreland basin. Hundreds of meters of sediment of the lower and middle Solimões Formation, with a maximum thickness of ∼1000 m in Acre, were deposited at that time in the foredeep zone. The Altiplano and the Eastern Cordillera in Bolivia had reached no more than half of their modern elevation by 10 Ma (Gregory-Wodzicki, 2000). Higher rates of uplift/deformation and denudation prevailed in the Central Andes during the late Miocene. In the Peruvian Andes, deformation was recorded at ∼9 Ma and 6–4.5 Ma (Megard, et al., 1984). Recent interpretations suggest a continuous period of maximum deformation between 9 and 6 Ma (Nobret et al., 1996). Nevertheless in Bolivia some controversies can exist about the rates of uplifted and on the tectonic models (Garzione et al., 2007; Hartley et al., 2007), has been proposed that the Bolivian Altiplano suffered rapid uplift be- tween ∼10.3 and ∼6.8 Ma (Garzione et al., 2007; Ghosh et al., 2006). Sedimentation in the southernmost part of the Bolivian foreland (Fig. 15) was controlled and spatially restricted by processes associated with the crustal shortening related to the proximity of the Brazilian shield (Isacks, 1988) and the deformation advancing into the foreland basin (Kennan., 2000). Late Miocene (∼9–6 Ma) sediments reached the eastward area of the Iquitos structural high (Fig. 1), where they probably formed a condensed sequence, but the sedimentation reached, in general terms, the Purus arch to the east and, moreover, the sediments peripherally overlapped the arch to the east. This thinning could indicate the possible position of a distal depositional zone at that time or a forebulge position in the basin. However, this interpretation is speculative because of the scarcity of well-calibrated temporal data in that area. The sedimentary response to the Andes–Subandean deformation in southwestern Amazonia during the late Miocene was deposition of a complex continental sedimentary sequence at times of a low global sea level (time equivalent of the Huayquerian– E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 119

Fig. 15. Situation of South America during the Late Miocene, indicating Late Miocene units in Venezuela, Brazil and La Plata basin. Note that some areas cannot be correlated with the Solimões Formation contrary to the suggestions of some authors because of the non-availability of the sedimentary record there (Chaco, eastern Amazonia, and part of the Colombian Llanos). The Late Miocene (Huayquerian–Mesopotamian) is a time of widespread continental conditions across South America.

Mesopotamian mammal fauna). Southwestern Brazilian Amazonia belts—all of which were regulated mainly by avulsion processes was connected directly to the Andes chain and a megadepositional (Fig. 14). subsiding system was emplacing sediments of the uppermost levels of A wide connection between the Amazon basin and the proto- the Solimões Formation (as identified in Acre; Figs. 15 and 16). Large Orinoco system, draining to the north and flowing into the Maracaibo river-dominated mega-fans coming from the Andes spread over a basin (cf. Hoorn, 1994b), would not have been possible (see subsiding tropical plain containing shallow lakes, swamps, and fluvial suggestions by Dias de Gamero, 1996). The widespread record by

Fig. 16. Paleogeographical reconstruction of sediment and water influx in the Amazon basin during the late Miocene and from the Pliocene to present. (A) The area was an active sedimentary subsiding basin during the Late Miocene. Western Amazonia received sediments directly from the Andes (dotted area. The eastern limit of the Solimões basin is indicated with a dashed line. The incomplete Amazon River system was just draining the cratonic-platforms area located to the East. (B) After the reorganization of the basin, during the transition between the end of the Miocene and the early Pliocene, the rivers of Southwestern Brazilian Amazonia (Purus, Jurua, and Javarí basins) became lowlands rivers without contact with the Andes chain and the Peruvian basins (Ucayali and Madre de Dios) were reorganized into the form we observe them today. The Amazon system became totally integrated as a transcontinental fluvial basin. 120 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124

Hoorn (1993, 1994a) of older Paleozoic and Tertiary deposits Relations between the Amazon fluvial system and the evolution of (Araraquara Formation and early Miocene sediments of Mariñame) the Amazon sea fan in the Atlantic Ocean can be also outlined. In a produces complications for the hypothetical connection (Fig. 15). In recent paper Figueiredo et al. (2009) suggested a low sedimentation the Colombian Llanos, transgressions during the Miocene are rate in the Amazon deep-sea fan between ∼11.8 and 6.8 Ma as a associated with the Leon Formation, of no more than Middle Miocene consequence of sediment trapping activity in the continental basins. age in Colombia and Eocene age in Venezuela (Cooper et al., 1995). They estimated the ages of the Amazon deep-sea fan sediments from This transgression remained north of ∼3°N (Cooper et al., 1995). the study of nanofossils in two wells, and interpreting the deep-sea Taking this record into account, there is a lack of marine late Miocene fan sedimentary record, they speculate on the environmental scenario (b10 Ma) deposits in the Colombian Llanos, but approximately 3000– of continental Amazon basin, where the Solimões Formation was 3500 m of late Miocene continental clastic sediments of the Guayabo being deposited. Figueiredo et al. (2009) do not mention that their Formation, deposited from about 10 to 2 Ma (Fig. 14). proposal is in large agreement and temporally overlaps significantly Additionally, the absence of Caribbean Cyprideis species in the with the proposed ages of the Solimões Formation by Latrubesse Amazon basin shown that a connection and marine transgressions (1992); Latrubesse et al. (1997), Latrubesse et al. (2007) and Cozzuol from the Caribbean Sea inland towards the Amazon cannot be (2006) who suggested that the sediments of the Solimoes Formation sustained (Muñoz-Torres et al., 2006). were mainly trapped during the Late Miocene in the Amazon basin The late Miocene sediments of the Solimões Formation cannot be and with the paleogeographic model that proposed a similar correlated with the Barreiras Formation, described in the Bragantina chronology for the organization of the Amazon fluvial system and Para platforms on the Atlantic coast; an unconformity (Fig. 15)is (Latrubesse 1992; Latrubesse et al., 1997, 2007). postulated in that area during the late Miocene (Rossetti, 2001). The low rates of sedimentation recorded between ∼11.8 and 6.8 Ma No continuity of foreland sedimentation can be demonstrated to by Figueiredo et al. (2009) can be interpreted differently in terms of the the south, in the Bolivian plains, because the scarce to inexistent paleogeographic evolution of the basin and sediment source through sedimentary and paleontological late Miocene records in that region the drainage area. The Middle-Upper Miocene sediments identified by are (Fig. 15). The only sediments with fossils of late Miocene Figueiredo et al. (2009), in our opinion, do not represent a transcon- (Huayquerian–Mesopotamian) age in Bolivia are those of the Solimões tinental Amazon river but just a dominant “cratonic-platform sediment Formation in Pando, near the Bolivian border with Acre, Brazil. The sourced” river (Fig. 16). Figueiredo et al. (2009) assume that during the distribution of late Miocene sediments recorded in the Amazon basin Early to Middle Miocene the deep-sea fan received sediments from the seems to be associated with the dynamics of the Central Andes and the nearest Proterozoic terrain to the Amazon mouth, (the Maroni– distribution of the Solimões Formation in Amazonia coincides with the Itacaiunas province-1.95–2.2 Ga old, middle Paleoproterozoic, Brito plate subduction segment between 2°S and 15°S. The sedimentological Neves 2002) and, they also interpret that during the next period (11.8– discontinuity of late Miocene sediments to the south of ∼13°S in 6.8 Ma) the clastic sediments were sourced from rivers draining the Rio Bolivian territory (Marshall and Sempere, 1993) can be interpreted as Negro–Juruena province (1.8–1.55 Ga old, late Proterozoic, Brito Neves a natural limit of the Tertiary Amazon basin that results from the 2002) and the river became a transcontinental river. What happened prominent effect of the Bolivian Orocline. While sedimentation was with the two huge largest areas of Proterozoic geologic provinces, occurring in the Andes foreland basin of western Amazonia during the (Central Amazonia and Tapajos Ventuari) which are located in between late Miocene (Figs. 1 and 15), shortening was occurring in Bolivia along the Maroni–Itacaiunas and Rio Negro–Juruena provinces is not the orocline zone in the direction of the Brazilian shield, which explained. It is important to remark that a good part of the large constrained sedimentation to the south. This period coincided with the “cratonic-platform” Amazon drainage basin tributaries from north and eastward-migration of the foreland system in Bolivia and the existence south, such as Negro, Trombetas, Xingu and Tapajos Rivers (Latrubesse of the late Miocene Intra-Chaco-discordance (Gubbels et al., 1993) et al, 2005) are developed, on Central Amazonia and Tapajos–Ventuari produced by the forebulge migration (DeCelles and Horton 2003) and geologic provinces. It is also important to remember that these four coincident with the southernmost sedimentation of the Bolivian Chaco basins combined represent nearly 30% of the total drainage area of the where the uppermost continental deposits of the Yecua and the Amazon basin today. That means, during times previous to the Pliocene Tariquia Formation (Uba et al., 2007) were deposited. The uppermost these basins combined represented good part (roughly estimated in deposits of the Yecua Formation, assigned to the Late Miocene (∼7 Ma) ∼90%) of the drainage basin of the ancestral Amazon basin discharging (Hulka et al, 2006), are continental deposits characterized by to the Ocean. freshwater calcareous microfauna (Nicolaidis and Coimbra, 2008). Terrigenous sediments become important in the deep-sea fan The overlying Tariquia Formation correlated with good part of the since the end of the Miocene and/or beginning of the Pliocene. The age Huayquerian (Hulka et al., 2006) consists of up to 3800 m of fluvial of 6.8 Ma indicated for Figueiredo et al. (2009) for the increasing sediments, mainly sandstones and brown mudstones. At this time sedimentation rates in the deep-sea fan is 6.8 Ma, is almost the same (ca.7.9–6Ma) a fourfold increase (from 130 to 628m/m.y.) in age postulated for us for end of the sedimentation of the Solimõs sediment accumulation rates in the foreland is related to the Formation inland (6.5 Ma which represents the end of the Huayquer- deposition of Andean-derived coarsening and thickening-upward ian mammal age). sandstone-dominated Tariquia strata (Uba et al, 2007). Although had been tempting to correlate the paleogeography of A renewed Andean uplift of the Central Andes (2°–15°S), which the Amazon basin with the terrigenous hemipelagic deposits from produced fast uplift of the Eastern Cordillera, seems to have happened ∼5–4.5 Ma in the Ceara Rise, close to the Amazon cone (Harris and in a relatively recent time, as suggested by the well-developed Mix, 2002) the correlation is risky. These authors related the unconformities recorded mainly during the Pliocene (∼2–4 Ma) increasing sedimentation to the reorganization of the Amazon fluvial (Nobret et al., 1996). This supports the proposed age (∼5 Ma) of the system and its definitive connection with the Atlantic Ocean. But this shallowing of the subduction angle (3–15°S flat slab region) of the assumption needs to be considered with caution. The Ceara Rise wells plate (Jordan et al., 1983) and with the thick-skinned deformation in studied by Harris and Mix (2002) and Dobson et al. (2001) are located the Subandean foreland basin sedimentary sequences, with base- distally and to the south of the Amazon deep-sea fan and under the ment uplifts in the Divisor/Contamana Ranges, which affected older river-coastal influence of one of the largest fluvial system of South Cenozoic units such as the Ramón Formation as well as the America, the Tocantins–Araguaia fluvial basin. The Tocantins– sediments of the Solimões Formation. This final uplift may have Araguaia basin currently drains nearly 700,000 km2 of central Brazil had a first-order control for the reorganization of the Amazon fluvial before reaching the Ocean through the southern border of the Marajo basin. Island. This system along with local sediment sourced rivers from the E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 121 east coast watersheds could have been main agents providing Solimões Formation. The thickness of the late Miocene sediments is sediments to the Ceara ridge. variable, increasing in the direction of the depocenter of the basin to As previously postulated by Latrubesse (1992) and Latrubesse et the southwest near the Acre–Amazon states border. al, (Latrubesse et al., 1997, 2007), it is during the early Pliocene that The evolution of the basin during the late Miocene is related to the the Amazon basin acquires its present geographic appearance. The tectonic behavior of the Central Andes (∼2°S–15°S). The late Miocene watershed between the Ucayali system and the southwestern Solimões Formation sediments were deposited in a subsiding basin Amazonia lowland rivers (Purus, Jurua and Javarí basins) and and in a continental environment. The facies analysis, fossil fauna Peruvian tributaries of the Madeira River (Madre de Dios River content (vertebrates and mollusks), and palynological record indicate basin) were organized at that time (Latrubesse Rancy, 2000) and the that the environment of deposition was dominated by avulsive rivers lowlands of southwestern Amazonia stopped receiving sediments associated with megafan systems, flood basins (swamps, lakes, directly from the drainage of Andean basins (Fig. 16). Both the Ucayali internal deltas, and splays), and soils developed on flat dry areas, in to the north and the Madre de Dios drainage systems have been an environment dominated by grasslands and gallery forests typical of collecting the water and hundreds of millions of tons of sediments a tropical to subtropical environment (wet-dry seasonal climate). produced by the Andes that flow downstream into the Amazon river Late Miocene sediments were studied westward of the Iquitos arch system through the Solimoes and the Madeira rivers and finally to the up to the border of Brazil with Peru (Divisor Ranges) and Bolivia Atlantic Ocean since the Pliocene. (Pando block). More detailed studies are needed eastward of the At that time, southwestern Brazilian Amazonia has ceased to be an Iquitos structural high in order to date the uppermost levels of the effective sedimentary basin but is, instead, an erosional area that Solimões Formation. However, is clear that the Solimões Formation contributes sediments to the Amazon fluvial system (∼100 million reached and partially overlapped the Purus arch to the west. The tons/year of suspended load derived to the Amazon from the Purus, Purus Arch was in general terms the eastward limit of the basin during Juruá, and Javarí rivers (Filizola, 1999). Today, approximately deposition of the Solimões Formation. At that time, the lower Amazon 1000 million tons of suspended sediments are being carried annually acted as a complex cratonic/platform basin discharging water and by the Amazon to the Atlantic Ocean, approximately 900 million tons sediments to the Atlantic Ocean. of which are sourced from the Andes (Fig. 16). About 5 Ma, a period of low angle of subduction was fully developed The lowlands of southwestern Amazonia have undergone uplift in the Nazca Plate between ∼3°S and 15°S (Jordan et al., 1983), and and dissection since the Pliocene with erosion dominating the deformation in the Subandean foreland produced tectonic reactivation regional landscape when compared to sedimentation than concen- of the Divisor/Contamana Ranges by compressional faulting. trated on relatively small prismatic sedimentary basins related to the During the end of the Miocene and the early Pliocene (∼6.5 Ma to main fluvial systems and the linear alluvial belts. Tectonic activity is a 5 Ma), the southwestern Brazilian Amazonia ceased to be an effective major factor affecting fluvial systems, controlling the direction and sedimentary basin and instead became an erosional area that size of the fluvial belts and inhibiting or promoting river migration. contributed sediments to the Amazon fluvial system. At that time, The presence of three main levels of Quaternary terraces, at 34–38 m, the watershed between the Ucayali system and the southwestern 15–20 m, and 8–12 m, is characteristic of several Southtwestern Amazonia lowland rivers (Purus, Jurua and Javarí basins) was Amazon rivers of the region, such as Juruá, Purus, and Acre (Fig. 2) and generated and the lowland fluvial systems of southwestern Amazonia terrace and alluvial deposit of different ages also spread on Peruvian became isolated from the Andes. On the basis of paleontological Amazon (Fig. 12). No widespread fluvial deposits and large flooded content (vertebrates and palynology) and facies analysis from river areas are recorded today in southwestern Brazilian Amazonia. banks, road cuts, and three wells, we have assigned the uppermost Flooded areas are restricted to relatively small active and locally levels of the Solimões Formation in western Amazonia, Brazil, to the subsiding basins in the Ucayali–Marañon system (Ucamara depres- Late Miocene. The vertebrate fossil record from outcropping sediments sion and others) (Dumont and Fournier, 1994) or along the flood- is assigned to the Huayquerian–Mesopotamian mammalian biozones, plains of large rivers as linear geomorphologic features. spanning 9–6.5 Ma. Additionally, we present results that demonstrate Finally, it is useful to enumerate some of the major puzzles that still that deposits in Peruvian Amazonia attributed to Miocene tidal remain in the region. For example, can we determine the precise age of environments are actually fluvial sediments that have been misinter- the lowermost levels of the Solimões Formation? Can we know more preted (both environmentally and chronologically) by several authors. accurately the age and environment of sedimentation of Ramon The entire Late Miocene sequence was deposited in a continental Formation? What is the relation between the northernmost records of environment within a subsiding basin. The facies analysis, fossil fauna the Solimões Formation and tectonic activity in the northern Andes? content, and palynological record indicate that the environment of Considering the proposed age of 6.4 Ma for the beginning of high rates deposition was dominated by avulsive rivers associated with megafan of sedimentation in the deep-sea fan of the Amazon (Figueiredo et al., systems, and avulsive rivers in flood basins (swamps, lakes, internal 2009), the minimum age of ∼6.5 Ma for the Solimoes Formation deltas, and splays). Soils developed on the flatter, drier areas, which (Cozzuol 2006; Latrubesse et al. 2007) and the age of 5 Ma for the main were dominated by grasslands and gallery forest in a tropical to changes in the tectonic of the Central Andes or Peru (Jordan et al., subtropical climate. 1983), a data gap of ∼1.6 Ma exists in the paleographic reconstruc- These Late Miocene sediments were deposited from westward of tions. It can be, of course, produced because of the imprecision in age the Purus arch up to the border of Brazil with Peru (Divisor Ranges) estimations in the different regions. In terms of paleogeography and and Bolivia (Pando block). Eastward of the Iquitos structural high, paleoecology, a good question to solve is to decipher a precise however, more detailed studies, including vertebrate paleontology, calibration of the Late Miocene–Pliocene events at different part of the need to be performed to calibrate with more precision the ages of the basin and to reconstruct with more accuracy the scenarios at this uppermost levels of the Solimões Formation. critical time, when the basin was strongly transformed. The evolution of the basin during the late Miocene is mainly related to the tectonic behavior of the Central Andes (∼3°–15°S). At approxi- 4. Conclusions mately 5 Ma, a segment of low angle of subduction was well developed in the Nazca Plate, and the deformation in the Subandean foreland Analysis of samples from river banks and road cuts in southwest- produced the inland reactivation of the Divisor/Contamana Ranges and ern Amazonia, as well as from cores from wells IAS 32-AM, IAS 27-AM tectonic arrangements in the Eastern Andes. During the Pliocene and IAS 19 AM, have allowed us to assign a late Miocene age southwestern Brazilian Amazonia ceased to be an effective sedimentary (Huayquerian–Mesopotamian) to the uppermost levels of the basin, and became instead an erosional area that contributed sediments 122 E.M. Latrubesse et al. / Earth-Science Reviews 99 (2010) 99–124 to the Amazon fluvial system. At that time, the lowland fluvial systems of Valley, Colombia. America Association of Petroleum Geologists Bulletin 79, 1421–1443. southwestern Amazonia (the Purus, Jurua and Javarí basins) become Cozzuol, M., 2006. The Acre vertebrate fauna: diversity, geography and time. Journal of isolated from the Andes by the newly formed north-flowing Ucayali South American Earth Sciences 21, 185–203. system and south-east flowing Madre de Dios System. It was during the Cozzuol, M.A., Goin, F., De Los Reyes, M., Ranzi, A., 2006. The oldest species of Didelphis fl (Mammalia, Marsupialia, Difdelphidae), from the Late Miocene of Amazonia. early Pliocene that the Amazon uvial system integrated regionally and Journal of Mammalogy 87, 663–667. acquired its present appearance, and also when it started to drain water Czaplewski, N.J., 1996. Opossums (Didelphidae) and bats (Noctilionidae and Molossidae) and sediments on a large scale to the Atlantic Ocean. from the Late Miocene of the Amazon Basin. Journal of Mammalogy 77 (1), 8494. DeCelles, P., Horton, B., 2003. Early to middle Tertiary foreland basin development and the history of Andean crustal shortening in Bolivia. GSA Bulletin 115, 58–77. Acknowledgments Dias de Gamero, M.L., 1996. The changing course of the Orinoco River during the Neogene: a review. Palaeogeography, Palaeoclimatology, Palaeoecology 123, 385–402. We thank especially our paleontologist colleagues from the Dobson, D.M., Dickens, G.R., Rea, D.K., 2001. Terrigenou sediments on Ceara Rise. A Cenozoic Record of South American orogeny and erosion. Palaoeogrogaphy, Universidade Federal do Acre, J.P Souza Filho, A.Ranzy, R. Negri, J.C. Palaeoclimatology, Palaeoecology 165, 215–229. Bocquentin, and E.G. Silva for discussions and for many years of Dumont, J.F., Fournier, M., 1994. Geodynamic environment of Quaternary morphos- collaborative field work in southwestern Amazonia. We also want thank tructures of the Subandean foreland basins of Peru and Bolivia: characteristics and study methods. Quaternary International 21, 129–142. Prof. Mario Assine for providing Landsat 5 TM images from the Mato Farrell, K.M., 1987. 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